If you listen to the SoundCloud interview that was linked earlier, you'll get some insight into the design. The designers had good goals but they do reveal a little ignorance of the market they're entering. They're clearly not fully aware of what's out there, but I don't think that matters much insofar as the design is concerned.
I think they missed their goal for ease of use slightly. As Vape Critic points out, there is some technique to learn, which is what I expected based on the design. It's not just push-and-hit, but it also doesn't look like it will take much to figure out.
I like that they've limited the temperature (no combustion) but I think that 400°F (~204°C) is a bit on the low side. People wanting (or needing) high temperatures will not get what they want here. On the other hand, I prefer low temperature vapour so for me it will be just fine.
I prefer devices that provide heat-on-demand , so the Firefly is appealing to me. They don't think it has any competition but I'm sure they've never seen (much less used) the FV, which can deliver as much power as the FF to the heater, but doesn't limit it. (That's both a blessing and a curse, because it's easy to hit combustion in the FV.) To be honest, I'm not sure how much value there is to being able to deliver 40W but capping the temperature as low as they have. Also, Vape Critic doesn't mention how hot it gets, but I have to think that the heater end of that thing gets pretty warm.
A proprietary battery is a negative, and it will be interesting to see what the battery lifetime will be. If they've revealed what the battery chemistry is, I missed it.
Overall though, for me the positives outweigh the negatives by a good margin. I'm looking forward to trying one of these.